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| Home > Publications database > Rastertunnelmikroskopische und -spektroskopische Untersuchungen an strukturierten Ge/Si(111)-Oberflächen |
| Home > Publications database > Rastertunnelmikroskopische und -spektroskopische Untersuchungen an strukturierten Ge/Si(111)-Oberflächen |
| Dissertation / PhD Thesis/Book | PreJuSER-36551 |
1999
Forschungszentrum, Zentralbibliothek
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/20535
Report No.: Juel-3707
Abstract: The growth of two dimensional (2D) Ge islands on differently structured Si(111) surfaces is studied by Scanning Tunneling Microscopy (STM) . If the surface is structured by the 60 x 60-reconstruction of the Sb-terminated Ge Stranski-Krastanov wetting layer, there is no correlation between this reconstruction and the nucleation sites of the Ge islands, but even in submonolayer deposition, the 60 x 60-reconstruction is strongly distorted . By evaluating the dependence of the 2D-island density on the growth temperature and applying standard nucleation theory a decrease of the appearent barrier height for diffusion on the Sbterminated surface in comparison to diffusion on the wetting layer without Sb is found, which is in agreement with ab initio calculations. The increase of the observed absolute island density can be explained by an activation barrier for the Sb/Ge-exchange wich was also predicted by ab initio calculations . Further deposition of Ge on the Sb-terminated wetting layer leads to 3D island formation and a rough surface . At higher coverages above 14 monolayers (ML) and in a certain temperature range between 580 °C and 630 °C the Sb/Ge/Si(111)-layer forms a smooth surface on atomic scale which is not observed without Sb. This is attributed to the formation of the dislocation network at the Si/Ge-interface during growth in the low coverage regime . In the proposed model the dislocation network is formed in the highly stressed trenches between the dislocation free islands at coalescence coverage . At temperatures above 580 °C the dislocations formed in the trenches are free to move to the interface to relax the misfit stress . In the temperature regime above 640 °C the islands are too big at coalescence and the misfit stress is released by dislocation formation . The dislocation network at the interface causes a regular strain field and a height undulation of 0 .5 A amplitude at the surface of 20 ML thick films . This can be measured by STM . This undulation is accompanied with a modulation of the lateral lattice constant, which leads to a regular array of preferred nucleation sites for Ge islands . The lateral lattice constant at the undulation maxima is more silicon like, while at the minima it is closer to the Ge bulk value . At growth temperatures of 350 °C we found a nucleation density at the minima, which is 3 times larger then at the maxima. At higher temperatures this ordering is disturbed by a prefered nucleation at domain bounderies and at step edges . Due to the valence band offset between Ge and Si, Ge islands on a Si/SiGe/Si(111) heterostucture can be used as quantum dots for hole states. The Si interface layer forms a tunneling barrier for hole transport . With the tunneling gap as a second barrier, Scanning Tunneling Spectroscopy (STS) measurements on the islands can probe confinement or coulomb blockade behaviour . Room temperature STS measurements did not show quantum dot effects of the observed islands with lateral dimensions of several 100 A. A rough theoretical estimation of the difference of the discrete energy levels of this sized island results in energy spacing in the order of 10 meV, which could not be resolved at room temperature. Further investigation should be done with Low Temperature STM .
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